The present disclosure relates to a thin-film transistor making use of a poly-crystal oxide semiconductor in the channel layer thereof, a display apparatus including the thin-film transistor and an electronic apparatus employing the display apparatus.
In recent years, display apparatus are designed to have large sizes and high frame rates. In addition, with development of a 3D display apparatus, it is absolutely necessary to add functions to a display device as well as its peripheral devices and to improve the performances of these devices. At the present day, amorphous silicon hydride (a-Si:H) is used to obtain a TFT (Thin-Film Transistor) offering more stability than a TFT which makes use of a covalent semiconductor such as silicon (Si) or gallium arsenide (GaAs). Since amorphous silicon hydride (a-Si:H) can be deposited at low temperatures, amorphous silicon hydride (a-Si:H) satisfies cost and process-temperature restrictions. However, amorphous silicon hydride (a-Si:H) has a low-mobility shortcoming. That is, field-effect mobility <2 cm2/Vs. For this reason, there has been aggressively developed a next-generation TFT material which has a high mobility and is proper for large-area applications.
Among such TFT materials, there is a promising TFT material referred to as an AOS (amorphous oxide semiconductor) which draws much attention. The AOS can be made by making use of all but the same process facilities as the amorphous silicon hydride (a-Si:H). That is to say, the AOS can also be deposited at low temperatures. In addition, since processes such as a laser annealing treatment are not required, the AOS can be produced at a low cost and is proper for large-area applications. On the top of that, the mobility of the AOS can be increased to about 10 cm2/Vs. At the present day, a 37-inch LCD (Liquid-Crystal Display) apparatus and a 12-inch OELD (Organic Electro Luminescence Display) apparatus which are both provided with TFTs made from the AOS are reported to be still at a stage of development.
In order to put the TFT made from the AOS to practical use, however, it is absolutely necessary to improve the reliability of the TFT. In the AOS, a bond between a metal atom and an oxygen atom is instable. Thus, the oxygen atom can be detached from the bond with ease. A typical example of the bond between a metal atom and an oxygen atom is an In—O bond. Thus, there are raised problems that high-concentration carriers uncontrollable due to the loss of the oxygen are introduced with ease and that the characteristics of the AOS change with ease. A typical change of the characteristics of the AOS is a shift of the threshold voltage Vth.
In order to solve these problems, there are provided some techniques. As a technique for solving these problems, an annealing treatment is carried out after a film creation process. By carrying out an annealing treatment after a film creation process, the number of instable metal-oxygen bonds can be reduced. As another technique for solving these problems, a protection layer is created on a channel layer made from the AOS. By creating a protection layer on a channel layer made from the AOS, it possible to prevent the oxygen from departing from the bonds. However, it is difficult to completely eliminate the changes of the characteristics. Thus, it is deemed to be absolutely necessary to substantially eliminate instable bonds between metal and oxygen atoms.
At the same time, there has been reported development of a poly-crystal oxide semiconductor making use of elements of the III and V groups such as InZnO or In2O3. For more information on this development, the reader is advised to refer to documents such as Japanese Patent Laid-open No. 2008-311342. In the known semiconductor of the III-V groups, a bond is created on an sp3 mixture locus. Thus, the known semiconductor raises a problem that large variations of the carrier transport characteristic and the like are generated if a crystal grain boundary exists. In the case of a poly-crystal oxide semiconductor, on the other hand, the conductivity of carriers is determined by a 5s locus. Thus, there are only few effects caused by the grain boundary as effects of carrier scattering and the like. In addition, the poly-crystal oxide semiconductor has merits that the mobility of carriers is higher than that of the AOS and that the oxygen loss which becomes a problem of the AOS is hardly incurred. The oxygen loss is hardly incurred because a 4-coordinate bond is created between a metal atom and an oxygen atom in a process of crystallization. A typical example of the 4-coordinate bond between a metal atom and an oxygen atom is an In—O bond.